This invention relates to a fluid compressor. More particularly, this invention relates to a fluid compressor either of the radial-flow centrifugal type or of the radial/axial, mixed-flow type having novel apparatus for inhibiting surging of fluid flowing in the compressor. The surge inhibiting apparatus is also effective to selectively vary both the fluid mass flow rate through the compressor and the pressure ratio which the compressor develops. Combustion turbine engines frequently include compressors of the two above-mentioned types to compress atmospheric air to sustain combustion powering the engine. Consequently, this invention also relates to combustion turbine engines.
A conventional method of inhibiting compressor surging is to permanently dispose a multitude of radially extending, variable-angle guide vanes in the inlet of a compressor. When the guide vanes are set to a zero angle of incidence fluid flowing in the inlet passes axially to the impeller. By collectively varying the angle of incidence of the guide vanes from zero to a determined angle a tangential velocity (swirl) is imparted to the flowing fluid. In comparison with axially flowing fluid, the swirling fluid decreases the angle of attack of the blades of the impeller. Because the angle of attack of the impeller blades is decreased, fluid surging is suppressed and the fluid mass flow rate and pressure ratio are decreased. U.S. Pat. No. 2,339,150, granted Jan. 11, 1944 to C. F. Codrington illustrates a blower control system having a multitude of variable-angle guide vanes.
Fluid compressors having variable-angle guide vanes have a number of recognized deficiencies. For example, because the guide vanes are always present in and obstruct the inlet all of the fluid flow through the compressor must pass between the guide vanes. Consequently, the guide vanes cause a pressure drop upstream of the impeller even when they are set to a zero angle of incidence. This undesirable pressure drop decreases the pressure ratio across the compressor. As a result, a larger impeller is required to accomplish the desired pressure ratio. Additionally, the actuating mechanism for the multitude of variable angle guide vanes is complex and expensive to manufacture.
Another conventional method of inhibiting compressor surging is to selectively recirculate a portion of the compressor discharge fluid back to the compressor inlet. The recirculated fluid is introduced tangentially into the inlet to impart a tangential velocity to the fluid flowing to the impeller. U.S. Pat. No. 2,660,366, granted Nov. 24, 1953 to H. Klein, et al., illustrates a compressor surge inhibitor recirculating a portion of the compressor discharge fluid.
As with compressors having variable-angle guide vanes, compressors recirculating a portion of the discharge fluid also have a number of recognized deficiencies. Among these recognized deficiencies is the reduction in compressor efficiency caused by recirculation of compressor discharge fluid. Additionally, the recirculated discharge fluid is warm because of compression so that it increases the temperature of the fluid flowing to the impeller; further decreasing compressor efficiency.